Optical add/drop multiplexing (OADM) devices, which have been introduced in land-based optical communication networks, have become applied to optical submarine cable systems. As a result, even optical submarine cable systems have been able to flexibly support a wide variety of network configurations. However, OADM functional in a typical optical submarine cable system is provided in a branching device installed on the seafloor. Accordingly, when OADM functional of an optical submarine cable system is to be changed after beginning of operation of the optical submarine cable system, a branching device needs to be raised from the seafloor onto land, and work such as replacing an optical filter needs to be performed depending on a changed network configuration.
On the other hand, in land-based optical communication networks, reconfigurable optical add/drop multiplexing (ROADM) devices have been widely used. An ROADM device allows a network configuration to be changed (reconfigured) after beginning of operation without replacing the OADM device. For example, PTL 1 describes an ROADM device that uses a wavelength selective switch (WSS). A WSS includes three functions: a demultiplexing function of separating input optical signals by a wavelength unit, a switching function for selecting demultiplexed optical signals, and a function of multiplexing selected optical signals. A WSS may further include a function of adjusting an optical signal level with each wavelength. PTL 2 describes an optical cross-connect device including a redundant configuration. In a typical ROADM device, WSSs are provided for each of an uplink and a downlink, and optical signals with predetermined wavelengths are distributed to trunk stations and branch stations.
FIG. 10 is a block diagram illustrating a configuration of a typical optical submarine cable system 90. The optical submarine cable system 90 includes an ROADM device 900 and landing stations 101 to 103. The ROADM device 900 is a branching device installed on the seafloor and is connected to the landing stations 101 to 103 installed ashore. The landing stations 101 to 103 are terminal stations that terminate optical submarine cables. Wavelength division multiplexing (WDM) optical signals (hereinafter referred to as “WDM signals”) are transmitted between the landing stations 101 to 103 and the ROADM device 900 by using the optical submarine cables.
Optical signals with some wavelengths among WDM signals transmitted and received by the landing station 101 and the landing station 102 are used for communication with the landing station 103. The landing stations 101 and 102 are also referred to as trunk stations. The landing station 103 is a terminal station that terminates a link (branch link) that branches from a link (trunk link) between the trunk stations and is also referred to as a branch station. In each drawing of the present application, a direction and a link from each of the landing stations 101 and 103 to the landing station 102 are referred to as the “up direction” and “uplink”, respectively, and a direction and a link from each of the landing stations 102 and 103 to the landing station 101 are referred to as the “down direction” and “downlink”.
FIG. 11 is a diagram illustrating an example of wavelength bands of WDM signals input and output at the ROADM device 900. A signal A and a signal B are transmission signals of the landing station 101. A signal D and a signal E are transmission signals of the landing station 102. The signal A is an optical signal transmitted from the landing station 101 to the landing station 102, and the signal B is an optical signal transmitted from the landing station 101 to the landing station 103. The signal D is an optical signal transmitted from the landing station 102 to the landing station 101, and the signal E is an optical signal transmitted from the landing station 102 to the landing station 103.
A signal A′ and a signal B′ are transmission signals of the landing station 103 to the uplink, and a signal D′ and a signal E′ are transmission signals of the landing station 103 to the downlink. The signal B′ is an optical signal transmitted from the landing station 103 to the landing station 102, and the signal E′ is an optical signal transmitted from the landing station 103 to the landing station 101. The signal A′ and the signal D′ are dummy signals. The dummy signal is added at the landing station 103 in order to keep optical power of a WDM signal input into an optical submarine repeater installed along the submarine cable within a given range in a system. The dummy signal does not include information to transmit.
The signal A, the signal A′, the signal D, and the signal D′ belong to the same waveband, and the signal B, the signal B′, the signal E, and the signal E′ belong to the same waveband. Further, the waveband to which the signal A belongs and the waveband to which the signal B belongs do not overlap with each other. Each of signals A, A′, B, B′, D, D′, E, and E′ may be one carrier (carrier wave) optical signal or may include a plurality of carrier optical signals. In the following description, for example, a WDM signal made up of the signal A and the signal B will be denoted as a signal AB. Similarly, a WDM signal made up of other optical signals, such as a signal D and a signal E, will be simply denoted using symbols such as D and E. Further, for example, the signal A is denoted as (A) and a signal AB′ is denoted as (AB′) in each block diagram.
Propagation of a WDM signal in the up direction in FIG. 10 will be described. A signal AB transmitted from the landing station 101 is branched at a coupler (CPL) 111 in the direction of the landing station 102 and in the direction of the landing station 103. On the other hand, a signal A′B′ transmitted from the landing station 103 and having a wavelength band similar to that of the signal AB is input into a WSS 112. The WSS 112 multiplexes and demultiplexes the input signals AB and A′B′ to generate a signal AB′. The generated signal AB′ is transmitted to the landing station 102.
The ROADM device 900 includes a control circuit 950 for controlling the WSSs 112 and 122. A ratio between a capacity of transmission from the landing station 101 to the landing station 102 and a capacity of transmission from the landing station 101 to the landing station 103 can be changed by controlling the WSS 112 to change wavelength bandwidths of signals A, B, A′, and B′. Similarly, in the down direction, a ratio between a capacity of transmission from the landing station 102 to the landing station 101 and a capacity of transmission from the landing station 102 to the landing station 103 can be changed by using a coupler 121 and the WSS 122.
FIG. 12 is a block diagram illustrating a configuration of another typical optical submarine cable system 91. The optical submarine cable system 91 includes an ROADM device 901 and landing stations 101 to 103. The ROADM device 901 is installed on the seafloor and is connected to the landing stations 101 to 103 installed ashore through optical submarine cables. The ROADM device 901 includes WSSs 131 and 132 and couplers 133 and 134 on uplinks. The ROADM device 901 includes WSSs 141 and 142 and couplers 143 and 144 on downlinks. The ROADM device 901 includes a control circuit 951 for controlling the WSSs 131 and 132 and WSSs 141 and 142.
An operation of the ROADM device 901 will be described with reference to FIG. 12. The WSS 131 separates a signal AB received from the landing station 101 into a signal A and a signal B and outputs the signal A and the signal B. The signal A is output to the coupler 133 and the signal B is output to the coupler 134. The WSS 132 separates a signal A′B′ received from the landing station 103 into a signal A′ and a signal B′ and outputs the signal A′ and the signal B′. The signal B′ is output to the coupler 133 and the signal A′ is output to the coupler 134.
The coupler 133 outputs a signal AB′ generated by coupling the signal A and the signal B′ to the landing station 102. The coupler 134 outputs a signal A′B generated by coupling the signal A′ and the signal B to the landing station 103.
An operation of the ROADM device 901 for optical signals in the down direction is similar to the operation described above. Specifically, the WSS 141 separates a signal DE received from the landing station 102 into a signal D and a signal E and outputs the signal D and the signal E. The WSS 142 separates a signal D′E′ received from the landing station 103 into a signal D′ and a signal E′ and outputs the signal D′ and the signal E′. The coupler 143 outputs a signal DE′ generated by coupling the signal D and the signal E′ to the landing station 101. The coupler 144 outputs a signal D′E generated by coupling the signal D′ and the signal E to the landing station 103. In this way, in the ROADM device 901, the signal A′B and the signal D′E are transmitted to the landing station 103.
FIG. 13 is a diagram illustrating an example of wavelength bands of optical signals input and output at the ROADM device 901. In the ROADM device 900, a signal A directed to the landing station 102 and a signal D directed to the landing station 101 are also transmitted to the landing station 103. In the ROADM device 901, signals received at the landing station 103 from the uplink and the downlink are a signal A′B and a signal D′E, respectively. These WDM signals do not include the signal A directed to the landing station 102 and the signal D directed to the landing station 101. The ROADM device 901 can therefore prevent the signal A and the signal D from being picked up at the landing station 103.